Hybrid Event Beds in the Proximal to Distal Extensive Lobe Domain of the Coarse-Grained and Sand-Rich Bordighera Turbidite System (NW Italy)
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This is a repository copy of Hybrid event beds in the proximal to distal extensive lobe domain of the coarse-grained and sand-rich Bordighera turbidite system (NW Italy). White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/118682/ Version: Accepted Version Article: Mueller, P, Patacci, M orcid.org/0000-0003-1675-4643 and Di Giulio, A (2017) Hybrid event beds in the proximal to distal extensive lobe domain of the coarse-grained and sand-rich Bordighera turbidite system (NW Italy). Marine and Petroleum Geology, 86. pp. 908-931. ISSN 0264-8172 https://doi.org/10.1016/j.marpetgeo.2017.06.047 (c) 2017, Elsevier Ltd. This manuscript version is made available under the CC BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. 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[email protected] https://eprints.whiterose.ac.uk/ Accepted Manuscript Hybrid event beds in the proximal to distal extensive lobe domain of the coarse- grained and sand-rich Bordighera turbidite system (NW Italy) Pierre Mueller, Marco Patacci, Andrea Di Giulio PII: S0264-8172(17)30254-4 DOI: 10.1016/j.marpetgeo.2017.06.047 Reference: JMPG 2981 To appear in: Marine and Petroleum Geology Received Date: 16 January 2017 Revised Date: 20 June 2017 Accepted Date: 28 June 2017 Please cite this article as: Mueller, P., Patacci, M., Di Giulio, A., Hybrid event beds in the proximal to distal extensive lobe domain of the coarse-grained and sand-rich Bordighera turbidite system (NW Italy), Marine and Petroleum Geology (2017), doi: 10.1016/j.marpetgeo.2017.06.047. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Idealized down-current axial sediment fairway stacking patterns S c. 60 km N 10 10 10 10 10 5 5 5 MANUSCRIP5 5 0 0 0 0 0 silt fsst csst gran cobb clay vfs msst vcs pebb clean sand argillaceous sand Proximal Distal ACCEPTED ACCEPTED MANUSCRIPT Hybrid event beds in the proximal to distal extensive lobe domain of the coarse-grained and sand- rich Bordighera turbidite system (NW Italy) Pierre Mueller¹*, Marco Patacci², Andrea Di Giulio¹ ¹ Dipartimento di Scienze della Terra e dell'Ambiente, Università di Pavia, Pavia, Italy. ² Turbidites Research Group, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, United Kingdom. • Corresponding author • Email-address: [email protected] (P. Mueller) Abstract The Upper Cretaceous Bordighera Sandstone of NW Italy is a coarse-grained, sand-rich elongated turbidite system (ca. 15 x 45 km in outcrop) up to 250 meters thick, interpreted to have been deposited in a trench setting. The siliciclastic succession interfingers with muddy calcareous turbidites, which become more abundant toward the lateral and distal domains. Bed type associations allow the distinction of a proximal channelized domain which transitions to a more distal lobe domain, characterized by abundant mudclast-rich sandstones and by bipartite and tripartite beds with a mud-rich middle or upper division (hybrid event beds). The transition between the proximal and distal domains occurs over a relatively limited spatial extent (ca. 5 km). The presence of lenticular bed-sets made up of coarseMANUSCRIPT grained and mud-poor sandstones throughout the distal domain suggests that distributary channels were present, indicating sediment bypass further down-dip toward the most distal and not preserved parts of the system. Hybrid event beds - commonly associated with distal and marginal fan environments such as fan fringes - are present throughout the lobe domain and extend for up to ca. 30 km in down-dip distance. They are more abundant in the proximal and axial depositional lobe domain and their appearance occurs within a short basin-ward distance from the inferred channel-lobe transition zone. Flow expansion at the termination of the channelized domain and the enhanced availability of cohesive substrate due to the presence of intra-basinal muddy calcareous beds are interpreted as the key controls on pronounced argillaceous sandstone distribution. The abrupt appearance and the persistent occurrence of such beds across an extensive domain have implications for characterizing bed-scale (sub- seismic) heterogeneity of deep-water clastic hydrocarbon reservoirs. ACCEPTED Keywords: sand-rich turbidite system, bed-scale heterogeneity, hybrid event beds, hyperconcentrated density flows, degree of flow confinement 1. Introduction The sediment gravity flow spectrum is traditionally regarded as comprising two end-members, namely cohesive ACCEPTED MANUSCRIPT flows (debris flows) and frictional flows (turbidity currents) (e.g. Lowe, 1982; Mulder & Alexander, 2001; Talling et al., 2012). Yield strength and grain interactions account for the dominant particle support mechanisms in debris flows, whereas the primary supporting mechanism of turbidity currents is represented by fluid turbulence (Sumner et al., 2009). In recent years, research efforts have been directed at better understanding flow states intermediate between the two end-members and that share both laminar and turbulent flow properties (e.g. Baas et al., 2009; Southern et al., 2015). Flume experiments (e.g. Baas et al., 2009; 2011) reveal that the concentration of clay within the flow plays a crucial role on emerging rheological flow heterogeneity as it dampens turbulence. Deposits that record both or intermediate flow characteristics commonly comprise a roughly two-part arrangement of a lower mud-poor sandstone portion and an upper mud-rich sandstone and have been documented in numerous studies from various settings (e.g. “slurry flows” of Lowe & Guy, 2000 and Sylvester & Lowe, 2004; “linked debrites” of Haughton et al., 2003; “hybrid event beds” of Haughton et al., 2009; “transitional flow deposits” of Kane & Pontén, 2012; “matrix-rich sandstones” of Terlaky & Arnott, 2014). As bed-scale heterogeneity has a negative impact on both lateral and vertical sandstone permeability, the occurrence of such deposits can result in sudden and unexpected reduced hydrocarbon production efficiency (e.g. Sylvester & Lowe, 2004; Amy et al., 2009; Fonnesu et al., 2015; Porten et al., 2016). Consequently, in the last decade studies on turbidite systems prone to this type of bed-scale heterogeneity increasingly came to the center of attention of both academic and applied research.MANUSCRIPT Hybrid event beds (HEBs) are a type of deposits cha racterized by the presence of a mud-rich division sandwiched between clean sandstone divisions (Haughton et al., 2009), resulting in a reduced primary porosity and permeability in comparison to classic turbidite sandstones. This enhanced mud content is interpreted to result from progressive clay entrainment during flow evolution (see also Haughton et al., 2003; Amy & Talling, 2007; Davis et al., 2009; Fonnesu et al., 2016). According to Haughton et al. (2009), the idealized deposit of such beds displays five divisions that tend to show variable individual thicknesses. A basal, structureless graded or ungraded sandstone (H1) that gradually passes into a banded sandstone (H2) which exhibits alternating cleaner paler sands and darker argillaceous sandstone bands, a chaotic, debritic argillaceous sandstone subdivision (H3) with variable concentrations of mud clasts and sheared sand patches, a fine grained, laminated mud-poor sandstone (H4) and a clayey mudstone cap (H5). Nevertheless, a single event bed does not necessarily compriseACCEPTED all of the mentioned divisions (e.g. Talling et al., 2012; Fonnesu et al., 2017). Hybrid event beds and other deposits characterized by an argillaceous sandstone division have largely been reported from distal environments of tectonically active lobe-dominated sand-rich and mud-rich submarine fans (e.g. Haughton et al., 2009; Hodgson et al., 2009; Muzzi Magalhaes & Tinterri, 2010; see review in Fonnesu et al., 2015). Moreover, hybrid event beds have been documented in proximity to basin margins, where flows are interpreted to decelerate rapidly through interaction with the laterally confining slopes (e.g., Barker et al., ACCEPTED MANUSCRIPT 2008; Patacci et al., 2014). Nonetheless, the occurrence of such beds has also been recognized in more proximal environments of largely unconfined turbidite systems, specifically in channelized realms located at the toe of slope of deep-sea fans (Henstra et al., 2016) or in the immediate downstream vicinity of the channel- lobe transition zone (Terlaky & Arnott, 2014). This paper presents data on the Upper Cretaceous Bordighera Sandstone